Coffee drinking may be associated with reduced risk of endometrial cancer; however, prospective data are limited. Further, it is not clear whether any association between coffee and endometrial cancer differs according to coffee caffeine content. The association of coffee drinking with incidence of endometrial cancer was evaluated among 226,732 women, aged 50-71, enrolled in the NIH-AARP Diet and Health Study who completed a baseline epidemiologic questionnaire. Following a mean 9.3 years of follow-up, data were available for 1,486 incident endometrial cancer cases. Cox proportional hazards models were used to estimate associations of coffee with endometrial cancer incidence. Sub-group analyses were performed according to smoking status, hormone therapy use (HT) and body habitus. Coffee drinking was inversely related to incidence of endometrial cancer (hazard ratio [HR] comparing drinking of >3 cups/day versus no cups = 0.64, 95% CI, 0.51-0.80; P(trend) = 0.0004). The association of coffee with endometrial cancer risk was apparent for consumption of both regular (HR per cup = 0.90, 95% CI, 0.86-0.95) and decaffeinated coffee (HR per cup = 0.93, 95% CI, 0.87-0.99). The relation of coffee with endometrial cancer incidence varied significantly by HT use (P(interaction) = 0.03) with an association only apparent among HT-never users (HR comparing drinking >3 cups/day versus no cups = 0.54, 95% CI, 0.41-0.72; P(trend) = 0.0005). Endometrial cancer incidence appears to be reduced among women that habitually drink coffee, an association that does not differ according to caffeine content.